This invention relates to CO.sub.2 detectors for use as part of a respiratory breath monitoring system for patients, usually in a hospital environment, and is specifically directed to a respiratory CO.sub.2 detector circuit which produces a high quality waveform from a cuvette for diagnostic purposes.
Still more specifically, this invention is an improvement over a respiratory CO.sub.2 detector disclosed by D. Raemer in a U.S. patent application Ser. No. 730,158, entitled "Respiration Detector" filed May 3, 1985, now U.S. Pat. No. 4,648,369.
The Raemer patent application disclosed a respiratory CO.sub.2 detector which included a sensor with a cuvette, a lamp as an infrared source, an infrared detector and amplifier for detecting the quantity of CO.sub.2 gas in the cuvette, and suitable electronic amplification circuitry. In this system, an increase concentration of CO.sub.2 gas in the cuvette resulted in an increased amount of energy absorption and less output from the infrared detector. A waveform of CO.sub.2 gas in the cuvette looks similar to a square wave. The high peaks represent approximately 5% concentration on exhalation and the valleys represent zero percent concentration on inhalation. The resulting detector output should closely approximate the inverse of the CO.sub.2 gas waveform, however, due to reasons to be described, the actual waveform was greatly distorted. Power for the infrared source was adjusted in the circuitry automatically depending upon the amount of attenuation in the optical path.
In this prior art scheme, no chopper or other reference was employed in order to keep manufacturing costs low. No attempt was made to preserve the quality of the CO.sub.2 output waveform because only an alarm signal level was desired to indicate low CO.sub.2 for the detection of apnea. Since no reference was employed, an artificial reference was developed by sensing the minimum CO.sub.2 concentration from each breath and controlling the lamp voltage to maintain a constant voltage output from the detector amplifier. Thus, more lamp voltage was applied when the optical path was attenuated due to water vapor and/or particulate contamination from the gas sample which tended to maintain the output voltage sensitivity to CO.sub.2 concentration in the dynamic breath waveform constant. The quality of the breath waveform was degraded because the system was designed to follow slow changes in CO.sub.2 concentration and slow changes in optical path contamination. This caused a "droop" in the CO.sub.2 waveform which was especially noticeable at lower breath rates of approximately four to six breaths per minute. In order to refer the waveform generated by the Baemen instrument to a baseline-suitable for reliable operation of a threshold detector, the waveform was connected to a high pass filter, which caused further distortion. This rendered the output waveform useless for diagnostic purposes. Nonetheless, the Raemer instrument did accomplish the purposes intended which were to (1) indicate by a flashing light when each expiratory breath occurred, and (2) sound an alarm when expiratory breaths were not present for a predetermined time.
When monitoring the concentration of CO.sub.2 in a patient's expired respiratory breath, it is important to have a high quality output waveform for diagnostic purposes.
This invention maintains the advantage of low manufacturing cost made possible by the Raemer invention, but enables the high quality CO.sub.2 waveform to be obtained for diagnostic purposes.
Therefore, an object of this invention is to provide a respiratory CO.sub.2 detector with
(1) a high quality waveform without additional requirements for references for zero CO.sub.2 or full scale CO.sub.2, PA1 (2) a high quality waveform in which the minimum CO.sub.2 concentration is referred to a stable zero baseline, PA1 (3) the ability to follow slow changes in CO.sub.2 concentration with minimal waveform distortion. PA1 (4) the ability to compensate for slow changes in optical path transmission with minimal waveform distortion.